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BAGHDAD, Iraq (CNN) -- With the country's parliamentary elections slated for Thursday and early voting already under way, a truck carrying what are believed to be fake ballots was detained in the Iraqi border province of Wasit, the U.S. military said Tuesday.

U.S. investigators were being sent to examine the ballots in Wasit, located on the Iranian border southeast of Baghdad.

The truck's driver was Iranian, as was the truck's license plate, said an Iraqi Interior Ministry source. Authorities are investigating reports that other vehicles with possible fake ballots have been trying to cross the border into Iraq.

Four killed

A homemade bomb killed four Task Force Baghdad soldiers Tuesday, the U.S. military said.

The four died while on patrol northwest of the capital, the military said.

Since the U.S.-led invasion in March 2003, the number of U.S. service members killed stands at 2,150.

It's the second time in a month that allegations have been made about abuses at an Iraqi government jail.

The top U.S. diplomat in Iraq said any sort of abuse is "unacceptable" and vowed that the U.S. would uncover exactly what happened at the facility, which he called overcrowded and in poor condition.

"I want to let the Iraqi people know that we are very committed to looking at all of the facilities. This is unacceptable for this kind of abuse to take place," Zalmay Khalilzad, the U.S. ambassador to Iraq, said, adding that the U.S. military and embassy staff had aided the investigation.

Khalilzad said only that the abuse uncovered "was far worse than slapping around." An Iraqi police source said fewer than 12 detainees were hospitalized after being beaten with cables or subjected to electric shock.

But that number fluctuated from source to source, with the Human Rights Ministry saying 13 detainees required medical treatment and Khalilzad reporting that more than 20 detainees were abused.

"We do not tolerate any violation of human rights for detainees," said Prime Minister Ibrahim al-Jaafari, who ordered inspections after last month's discovery in Jadriyah.

In the more recent case, Human Rights Ministry inspectors joined U.S. military personnel, representatives of the inspector general's office and other Iraqi ministries in a Thursday visit to the facility.

There, they questioned 625 detainees "about their treatment, the quality of services received and their health," according to a statement from the Iraqi Human Rights Ministry.

Sunni leaders have said that most of the detainees found at the facility in November were Sunnis -- a claim denied by Iraqi government officials. Sunni politicians have long alleged that Shiite militiamen have infiltrated Iraqi police and have abused Sunni prisoners.

Most Iraqi insurgent attacks have occurred in the so-called Sunni Triangle, where the nation's Sunni population is centered.

Khalilzad predicted earlier this week that large numbers of Sunnis -- some of them insurgents -- would vote.

"That is really the single most important development of the past several months politically," he said. "Sunnis seem to be developing confidence in the political process. They believe that their grievances can be dealt with politically."

In January, Sunnis largely boycotted the election for the transitional National Assembly. When the constitution was ratified in October, more Sunnis voted, and Khalilzad said Sunday that they are a potentially powerful minority bloc that could snare 40 to 55 seats in the next 275-seat assembly.

Sunni candidate killed

Gunmen in Ramadi on Tuesday fatally shot a Sunni Arab sheikh running for parliament, according to a Sunni political official.

Sheikh Muzhir Naji al-Dulaimi, a member of the Free Progressive Iraqi Party, was driving away from his home with two bodyguards when they were attacked in the al-Bakir neighborhood of Ramadi, the official said.

The city is the capital of Anbar province and was the site of a recent battle of U.S. and Iraqi forces against insurgents.

The sheikh's guards were critically wounded, and the official said he believed the gunmen were the same ones who killed Iraqi Islamic Party candidate Ayad al-Izzi a few weeks ago.

"But these cowardly acts will not prevent us from participating in the coming elections and to take an important role in the coming political process," he said.